Both air and water are optical media which are transparent to a range of wavelengths, and through which an intense laser beam can travel many tens of meters. Application of laser beams to such optical media can have many useful effects. For example, short-duration intense laser pulses have been used to generate acoustic pulses in water. See U.S. Pat. No. 7,260,023 to Jones et al., the entirety of which is incorporated by reference into the present application. Laser pulses have also been used to trigger high-voltage water switches, see J. R. Woodworth et al., “Laser-Induced Water Triggering,” Proc. 14th IEEE International Pulsed Power Conf, Dallas, Tex., 595 (2003) (“Woodworth I”); J. R. Woodworth et al., “170-kV Laser-Triggered Water Switch Experiments,” IEEE Trans. on Plasma Sci., 33, 2051 (2005) (“Woodworth II”); and J. R. Woodworth, et al., “Green-Laser-Triggered Water Switching at 1.6 MV,” IEEE Trans. on Dielectrics and Insulation 14, 951 (2007) (“Woodworth III”).
Both air and water have nonlinear optical properties when the laser beam is above a threshold power level. For example, both air and water are subject to the Kerr effect, wherein the index of refraction of the optical medium increases upon the application of an electric field associated with an electromagnetic wave such as a laser beam. The result of the Kerr effect acting on a laser beam with a centrally peaked intensity profile is beam self-focusing. This process increases the light intensity, thereby further increasing self-focusing in a self-reinforcing manner.
Both air and water also exhibit photoionization, in which the medium within a high-intensity light source becomes ionized. Various lens focusing configurations have been used to achieve laser intensities necessary for photoionization, such as simple lens focusing at large f/#, a compound lens, see Woodworth II, supra; an axicon, see Woodworth III, supra; or a cylindrical lens. Ionized molecules in an optical media decrease the index of refraction, and for typical centrally-peaked beam intensity profiles, the beam is defocused.
At high light intensity, Kerr-induced self-focusing and ionization-induced defocusing can combine and offset to result in the formation of an optical filament and an associated extended volume of ionized air or water. An optical filament is a light beam which propagates at high intensity and small radius for long distances, beyond the divergence distance normally determined by diffraction. Thus, generation and propagation of an optical filament can provide an improved method of photoionization along an extended conduction path.
The ionized channel associated with an optical filament can serve as a path of relative high conductivity, sufficient to guide an electrical discharge through that optical medium. For example, deionized water has a typical resistivity of 2 MΩ·m, which for a 1 square centimeter cross section column yields a resistance per length of 20 GΩ/m. In contrast, one initial measurement of an underwater filament yielded a filament diameter on the order of 10 μm and ionization fraction of 10−4, so that the filament resistivity per length was 50 MΩ/m, i.e., the filament was 400 times more conductive than a 1 cm2 conducting path through the surrounding deionized water. See S. Minardi, et al., “Time-resolved refractive index and absorption mapping of light-plasma filaments in water,” Optics Lett. 33, 86 (2008).
In addition, for laser-guided atmospheric discharges, the heating and rarefaction of neutral gas surrounding a filament can increase the electron mobility and augment the conductivity of the ionized channel. For example, atmospheric ultra-short pulse laser guided discharges were proposed, and initial experiments reported, as early as 1995. See Zhao, et al., “Femtosecond Ultraviolet Laser Pulse Induced Lightning Discharges in Gases,” IEEE J. Quant. Electr. 31, 599 (1995). Demonstration and time-resolved characterization of atmospheric discharges at NRL have been reported in Gordon, et al., “Streamerless Guided Electric Discharges Triggered by Femtosecond Laser Pulses,” Phys. Plasmas 10, 4350 (2003). For underwater laser guided discharges, heating-driven rarefaction and conductivity enhancement is even more dramatic, since there is a phase change from liquid to vapor which can correspond to a reduction in density of several orders of magnitude.
The laser-induced change in index of refraction scales with the optical media density, and affects the distance required for optical filament formation. The energy per length required to create an ionized channel also scales with the optical media density, and affects the distance the optical filament propagates before energy depletion.
Lifetimes of laser-generated liquid water plasmas have been reported to be on the order of 20 nanoseconds, based on preliminary measurements of broadband optical emission. See Yui, et al., “Spectroscopic Analysis of Stimulated Raman Scattering in the Early Stage of Laser-Induced Breakdown in Water,” Phys. Rev. Lett. 82, 4110 (1999).
Measurements of underwater filament parameters for a variety of conditions, including in laser-generated underwater vapor channels, are presently underway at NRL. Preliminary extended underwater laser propagation experiments at NRL suggest filament propagation up to a few meters in length, corresponding to a large upper limit for guided discharge lengths.